This invention relates generally to laser diodes and more particularly concerns a feedback system which compares an averaged train of pixel information against an averaged optical output power from the back photodiode in order to generate a feedback signal for adjusting the current of the laser diode to substantially correct the dependence of optical output power on laser junction temperature which is dependent on the pixel pattern.
Usually, in a scanning system, the light beam from the laser diode is modulated by a train of image pixel information. The pixel information is used to drive the diode and therefore stimulate laser emission where there is an On pixel. Since the emission of the light beam follows the pattern of the train of pixel information, this is called "modulation of the light beam by the train of pixel information". Hereinafter, pattern of the train of pixel information shall mean the pattern of the data change between the pixels of a train of pixel information; for example, from On to Off or from Off to On.
Due to the structure of the laser diode assembly, as the pattern of the train of pixel information changes, the temperature of the diode fluctuates which in turn causes the optical output power of the diode (intensity of the light beam) and the wavelength of the light beam, generated by the diode, to fluctuate. In a printing system, fluctuations of the optical output power cause fluctuations in the size of printed pixels which is not desirable.
The optical output power of a laser diode is highly sensitive to the temperature of the junction. When the laser diode is turned on, it heats up and typically within fifty to a few hundreds of microseconds causes its optical output power to settle down to a decreased output by several percents, a phenomenon called "droop".
In addition, droop is dependent on the pattern of the train of pixel information which is applied to the laser diode. Every time there is an On pixel, since the diode starts emitting light, the junction of the diode heats up. Due to this generated heat, the temperature of the diode increases. However, where there is an Off pixel, the diode does not generate any heat.
Therefore, the temperature of the diode depends on the pattern of the train of pixel information. As a result, the intensity of the light beam and therefore the size of the printed pixels depend on the pattern of the train of pixel information.
Minimizing the fluctuations of the optical output power would result in more accurate printed pages with improved image quality. Ideally, in order to keep the optical output power of the diode (intensity of the light beam) and the wavelength of the light beam generated by the diode constant, the temperature of the diode should be kept at a constant level. However, this requires the use of a thermoelectric cooler which is expensive and can not respond fast enough to fluctuations of the modulation duty cycle.
Typically, to reduce the fluctuations of the optical output power, a feedback system should be connected to the laser diode to generate a feedback signal in accordance with the fluctuations of the heat in order to adjust the current of the laser diode to change the optical output power as needed. Ideally, the feedback signal should be generated to correct within one pixel while the train of pixel information is being applied to the laser diode (real time).
However, high speed real time feedback systems have the disadvantages of being very expensive to construct, requiring extremely fast components to effectively measure and provide a real time response within a pixel. Therefore, some other methods must be found.
A conventional method of monitoring the optical output power of a laser diode measures the optical output power between the end of scan (EOS) and the start of scan (SOS). During this brief dead time between EOS and SOS, the optical output power is measured to generate a feedback signal in order to adjust the current of the laser diode. However, this method only measures the optical output power of the laser diode during the time that the train of pixel information is not applied to the laser diode.